1. Field of the Invention
The present invention relates to stop-and-display stations in document-handling equipment, whereat a document in the course of processing is rendered visible to a user. In particular, the present invention relates to a sensing apparatus for detecting the presence of a document in a stop-and-display station.
2. The Prior Art
In document-handling equipment such as check-encoding equipment, it is known to provide a stop-and-display station. The document or check has data automatically read therefrom as it moves along a track. The document or check is brought to a halt in a stop-and-display station for visual inspection by a user. Whilst stopped, details recovered from the document or check can be confirmed by the user and other data not automatically recoverable can be entered into the machine by the user. In the case of check-encoders, the most frequently entered information concerns the amount of the check which, being hand-written, cannot be the subject for automatic reading by the machine. Further, the machine displays those details which it has automatically read from the check and, if there is any discrepancy between what is apparent to the user upon visual inspection of the check and what the machine believes it has read, the user can correct any misread details by use of a keyboard.
The construction of a stop-and-display station in general consists in a transparent glass wall, and a rear support wall wherebetween a document can be moved and stopped in a track. when a document is in the stop-and-display station, it is customary to provide a sensor which, sensing the presence of a document, signals to the control system in the check-encoding equipment that a document is stopped in the stop-and-display station. The control system in the check-encoding equipment performs different functions dependently upon the position of the document in its path through the machine. In the earlier stages of the passage of a document through a check-encoder, the control system controls and monitors the automatic reading of characters written upon the document. When the document or check is in the stop-and-display station, the control system must prepare itself to receive new information and to receive correction to any characters, automatically read from the check or document, which may be in error. Thus, when a sensor senses the presence of a check or document in the stop-and-display station, the control system responds to indication from the sensor by preparing to receive information via a keyboard. The automatic acceptance of keyboard data if and only if a document or check is present in the stop-and-display station, eases the burden on the user and prevents erroneous entry of data at other times.
It is known to detect the presence of a document or check in the stop-and-display station by means of a reflective optical sensor. The reflective optical sensor comprises a light source and a photodetector. The optical sensor is situated in the support wall of the stop-and-display station so that it is behind the document and is visible through the transparent wall whenever the document or check is in the stop-and-display station. The light source sends a beam of light up to the rear of the document, which beam is reflected by the document onto the photodetector. The mean level of light received by the photodetector is compared with a reference level and, when the reflected light exceeds a predetermined limit, indication is given to the control system that a document is present.
Problems exist with this scheme. Firstly, when a document is not present in the stop-and-display station, the photodetector can still pick up ambient background lighting through the transparent glass wall wherethrough the document or check is to viewed. Should ambient intensities become too great, a false indication is provided of a document being present when, in fact, there is not document or check in the stop-and-display station. Again, when no document is present, the beam of light from the light source can be reflected from the transparent wall's inner surface onto the photodetector to give an erroneous document presence indication.
It is important in the operation of photodetectors to provide an acceptably high signal-to-noise ratio. The signal-to-noise ratio is defined as the amount of light increase incident upon the photodetector when a check or document is present compared with the incident light upon the photodetector when no document is present. Even if the above described mechanisms do not cause an erroneous indication of document presence to be generated, they have the effect of degrading the signal-to-noise ratio seen by the photodetector making it a matter of some difficulty to select the threshold of incident light intensity whereabove the decision is taken that a document is present. This alone can cause malfunctioning of the sensing system.
It is therefore desirable to provide a document-sensing system for a stop-and-display station in document-handling or check-encoding equipment which is resistant to the effects of ambient lighting incident upon the photodetector through the transparent display wall, and which is also resistant to the effects of reflection from the inner surface of the transparent display wall.
3. Summary of the Invention
Specular reflection is hereinafter defined as reflection of a light beam wherein the angle of incidence onto the reflective medium is equal to the angle of reflection from the reflective medium. Specular reflection is therefore reflection in the manner of a perfect mirror.
Scattered reflection is hereinafter defined as reflection wherein a beam of light, incident upon a scattering surface, is reflected in many directions at once. Scattered reflection is characteristic of rough surfaces and is ideally represented by the reflection of light, for instance, from matt white painted surfaces.
It is to be appreciated that no surface reflects one hundred per cent in either of the above defined manners. Smooth surfaces approximate more and more closely to specular reflection the more smooth the surface profile, whilst rough surfaces approach one hundred percent scattered reflective conditions.
The present invention consists in an apparatus for sensing the presence of a document stopped in a track in a stop-and-display station, said stop-and-display station including a first transparent wall wherethrough the document can be viewed and a second wall for supporting the document against said first wall, said apparatus comprising: a light source in said second wall for projecting a beam of light, intensity-modulated with a predetermined frequency, at a predetermined angle of incidence onto a document intermediate between said first and second walls; a photosensor, also in said second wall, positioned to intercept light from said beam scattered by the document without intercepting light from said beam specularly reflected by the document, said photosensor being operable to provide an output signal representative of the instant intensity of light incident thereon; a band-pass filter coupled to receive as input said output signal from said photosensor and operable to provide as output those components of its input lying within a predetermined range of said predetermined frequency; and a threshold detector, coupled to receive said output of said band-pass filter, and operable to provide output, indicative of a document being intermediate between said first and second walls at each instance of each cycle of said output of said band-pass filter exceeding a predetermined amplitude.
4. Brief Description of the Preferred Embodiment
In a preferred embodiment of the invention, a check is moved along a track by any known means between a first transparent wall and a second support wall. The document is preferably stopped in the track by means of a solenoid insertable and withdrawable pin.
A reflective photosensor is provided in the secnd support wall. The reflective photosensor comprises a light source and a photodetector, the light source and the photodetector being angled to view a common area on the check or document. The reflective photosensor assembly is angled relative to the second support wall such that the beam of light from the light source cannot be returned by specular reflection from the document or the inner surface of the first transparent wall onto the photodetector. The photodetector can only intercept a portion of the scattered reflected light beam returned from the rough surface of the check or document.
The intensity of the light source is modulated at a predetermined frequency by means of a clock generator circuit and a driver circuit. The intensity of the light source is preferably one hundred per cent modulated. The photodetector provides an output representative of the instant radiation intensity incident thereon, which output changes sufficiently quickly for the modulated scattered reflected light at the predetermined frequency to be detected. The output of the photodetector is coupled to a band-pass filter which provides as its output only those components of the output of the photodetector lying within a predetermined range of frequencies about the predetermined frequency. The output of the band-pass filter is coupled to a threshold detector which detects when the output of the band-pass filter exceeds a predetermined amplitude. The output of the threshold detector thus is indicative of more than a predetermined amount of scattered light from the light source reflected from a document being incident upon the photodetector. The output of the threshold detector is therefore indicative of a document being present intermediate between the first and second walls of the stop-and-display station.
The output of the threshold detector is a logically-true signal for each occasion that a cycle of the waveform from the band-pass filter exceeds the predetermined threshold levels. The threshold detector circuit provides a stream of logically-true output pulses at the predetermined frequency whenever a document is present. The output of the threshold detector circuit is latched by a latch to provide a steady latch output signal for provision to the system controller to indicate to the system controller that a document is present in the stop-and-display station.
The predetermined frequency and the range of frequencies about the predetermined frequency which the band-pass filter will pass are together chosen such that no frequency component of the ambient lighting will excite the threshold detector circuit. The predetermined frequency is chosen having regard to the commonly-used excitation frequencies for fluorescent lighting of 100 Hz and 120 Hz. The predetermined frequency is chosen such that no Harmonic of 100 Hz or 120 Hz falls within the predetermined range of the band-pass filter.
In order that endorsement and marks upon the rear of the document should not cause an erroneous resetting of the latch, there is provided a turn-off delay circuit. The turn-off delay circuit prevents the resetting of the latch indicatively of no document being present if, and only if, the threshold detector circuit has not provided an output for longer than a predetermined period. In the preferred embodiment described, the turn-off delay circuit counts the number of clock cycles from the clock generator circuit which drives the light source at the predetermined frequency, and resets the latch if, and only if, three or more clock cycles are received without receiving an output from the threshold detector circuit. Counting is achieved by propagation of a logical input condition between stages in a shift register where the last stage of the shift register is the latch. The latch has its condition altered only after the third clock pulse delivered to the shift register.
A light-emitting diode is provided for giving indication to the operator that a successful document detection has taken place.